I was first thinking that the anti slosh was to prevent when tank is low on gas you don't get a bubble in the fuel line and stall out. Baffles in the tanks that some have fabricated in the DIY area show these as part of the tank construction.
As for gas leaking out the cap, I remember the kind of metal gas cap with a fiber cone shape breather that was supposed to stop the gas from coming out the 70's era 3hp Briggs engine I have. I bought a newer model plastic Briggs mower gas tank. Sure it really does not have the aesthetic appeal that you want, but the cap I have no leaking with gas at close to 1/2 inch from the top.
This cap that breathes also does no have a hole in the center of the top of the cap. It some how has something around the edges of the cap, but the threads must be separate from it I think.
Do look at the kind of set up on Dirt Bikes that have the little tube comes out the ceter of the cap and go forward down into the steering column hole. It has to go like a foot higher than the tank in an arc and the gas won't slosh up that high. Again, yea the looks may not be desired.
Just what I've seen.